This invention relates to an electrical test probe and, in particular, to a probe which is employed in conjunction with the testing of semiconductor chip microsockets. As the size of integrated circuitry chips is reduced, package density increases, thereby reducing the area which probes may have as a target. Moreover, a reduction in device size increases the propensity toward damage of the contacts. Consequently, there exists a continuing requirement in the testing of integrated circuit technology for a test probe system which provides for verifiable contacts on small chip attachment pads, yet not requiring manual or optical alignment. Moreover, given the overall size in the devices under test, it is important that the total allowable positional system error be maximized to an acceptable level to eliminate the need for operator intervention or manual adjustment to accurately position probes on the pads under test.
Within the prior art, a variety of test devices have been proposed to perform different functions. These can broadly be classified as a defect analysis tester which utilizes two single point probes, a parametric resistance unit employing two bifuracted probes, a parametric capacitance unit which employs a shielded capacitance probe and a system for capacitance discharge short repairs which employs two single needle probes. Heretofore, none have been capable of performing multiple functions. The prior art is replete with examples of these various systems. U.S. Pat. No. 3,444,465 is a probe for graphic communications and utilizes a ballpoint pen-like refill as a center probe. A single centering spring is loaded with an outer shielding to eliminate shunt capacitance. The overall size of the system, and in particular that of the writing point 12, makes it generally inapplicable for use in testing chip microsockets. However, the use of a centering spring to provide loaded axial motion of the center probe is a design consideration in the development of test devices for use on fragile structures.
Another example of a spring loaded plunger single point contact system is found in U.S. Pat. No. 3,562,643. This device also employs a springloaded movement in conjunction with a flexing tip. Other examples are found in U.S. Pat. No. 4,423,373 and IBM Technical Disclosure Bulletin Vol. 21, No. 9, p. 3742, February 1979. U.S. Pat. No. 4,423,373 employs a coaxial center probe which is compression spring loaded. By utilizing dual contacts and having axial movement so biased, contact verification is possible. A somewhat different spring-loaded system is found in the IBM Technical Disclosure Bulletin wherein inner and outer coaxial probes are each independently spring loaded.
A plurality of conical point spring-loaded single contacts are disclosed in U.S. Pat. No. 4,164,704. The device therein is a test fixture for use in circuit boards with a test probe having a conductive spring and conductive contact cap which are slidably mounted between a fixed probe board member and plastic sheet used to urge the card under test to the probes.
A somewhat different arrangement is disclosed in U.S. Pat. No. 3,826,981 which employs a single probe which is fixed with an outer shield grounded. This probe allows for capacitance measurements to be made when the outer contact acts as a shield.
Examples of probes which may be clustered are disclosed in U.S. Pat. No. 4,209,742 and in commercial devices made by Pylon, the POGO series of probes, and those manufactured by Micromanipulator.
A significant shortcoming of these prior art devices is that they require precise manual alignment in the case of bifurcated probes in the range of 0.005 inch diameter in the case of very small chip microsockets. Test probes which utilize two needles having points in close proximity, such as 0.001 inch each utilize as a target approximately one half of the diameter of the microsocket pad. If the diameter is in the range of 0.005 inch, the total allowable system error is therefore .+-.0.002 inch. As can be appreciated, the allowable system error with these product devices is extremely small. Thus, discrete probes suffer from serious disadvantages. When testing semiconductor ship microsockets these problems are significant.